Solid state imaging devices having pixel arrays, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) devices, are commonly used in photo-imaging applications (e.g., camera). A CMOS imager circuit, for example, includes a focal plane array of pixel cells, each including a photosensor, for example, a photogate, photoconductor or a photodiode for accumulating photo-generated charge. Each pixel cell has a charge storage region, which is connected to the gate of an output transistor that is part of a readout circuit. The readout circuit connected to each pixel cell includes at least the output transistor, which receives photo-generated charges from the storage region and produces an output signal that is read-out through a pixel access transistor. In some imager circuits, each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
CMOS imagers of the type generally discussed above are generally known as discussed, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524 and 6,333,205, assigned to Micron Technology, Inc., which are hereby incorporated by reference in their entirety.
Most cameras, including digital cameras, have an auto focus feature in which scenes viewed by the camera can be focused automatically. The auto focus feature can be continuous or user enabled (i.e., by depressing a button).
Currently, in one known auto focusing technique a microprocessor of an auto focus camera scans the difference in intensity among adjacent pixels to determine the best focus position of the camera's lens. If the scene is out of focus, adjacent pixels have very similar intensities. As the microprocessor moves the lens, each of the pixels are scanned to determine if the difference in intensity between adjacent pixels has improved. The microprocessor is searching for the lens position in which there is a maximum intensity difference between adjacent pixels (i.e., the best focus position of the lens).
If a scene has multiple objects that are of varying distances from the camera, it is often difficult to find the best focus position of the lens. Many of the conventional methods for seeking the best focus position are based on the average sharpness score over a frame or one window. Usually such methods do not produce the best results for a scene with two or more objects that are different distances from the camera. Sometimes, the lens position giving the maximum average sharpness for the scene may not match either one of the best focus positions for the objects in the scene. Accordingly, there is a need and a desire for a method of auto focusing a camera and its lens for multiple object scenes.